Conventional catalytic converters utilize a ceramic monolith on which is deposited a catalyst which aids in the conversion of pollutants such as, carbon monoxide, nitrogen oxides (NO.sub.x), unburned hydrocarbons, etc. to carbon dioxide, water and nitrogen. However, this conversion is not efficient initially while the exhaust gases are relatively cold. To be effective at a high conversion rate, the catalyst, e.g., palladium, and surfaces with which the exhaust gases come in contact must be at a minimum elevated temperature; for example, 350.degree. F. for carbon monoxide and 570.degree. F. for volatile organic compounds. The "light off" or conversion temperatures will depend on the catalyst and contact area. Otherwise, the conversion of these pollutants to harmless by-products is low and cold-start pollution of the atmosphere is high. Once the exhaust system comes to its normal operating temperature, the catalytic converter is effective. Hence, it is to the achievement of early conversion of pollutants before reaching normal operating temperature and while low exhaust temperatures are encountered that the present invention is primarily directed. This invention provides a novel structure for an electrically heatable element in a catalytic converter, and a novel catalytic converter characterized by the presence of such an element.
The principle of elevating the temperature of a catalytic converter to a higher, more efficient operating temperature for start-up is not, per se, new. Reference may be had to U.S. Pat. No. 3,768,982 to Kitzner dated 30 Oct. 1973. In this patent, heat from a centrally located electric heater is transferred by conduction through a monolithic catalyst support to heat the catalyst to an optimum operating temperature. Reference may also be had to U.S. Pat. No. 3,770,389 to Kitzner dated 30 Oct. 1973 which discloses a central electrically heated core within a ceramic monolith, heat being transmitted by conduction to the catalyst contained in the openings of the ceramic monolith. The heating core is formed of metal sheets, one flat and the other corrugated, coated with alumina and also bearing a catalyst. The metallic core is heated electrically by virtue of its own electrical resistance.
Reference may also be had to the copending application of Cornelison et al Ser. No. 196,301, filed 20 May 1988 for discussion of an electrically heated catalytic core formed of corrugated thin metal strips having an aluminum oxide coating on the surface thereof.
Reference may also be had to the U.S. Pat. of Cornelison and Retallick, No. 4,711,009 dated 8 Dec. 1987 for details of the preparation of corrugated alumina coated thin metal strips having a catalyst deposited thereon. This patent is incorporated herein by reference thereto. Such strips may be utilized in the present invention.
The structures of the present invention are distinguished from the prior art in that the latter rely on a single corrugated thin metal strip, which is preferably accordion folded, to build up a metal monolith which is then inserted into a suitable housing. The devices of the present invention are formed of a plurality of strips electrically in parallel and secured at the ends thereof to suitable electrical contacts, and the entire unit formed to fit together with needed insulation into a suitable housing. In a preferred structure, a plurality of strips about 1" wide and 14" to 20" long, corrugated, wash coated with alumina, dried, and catalyst applied, are collected, stripped at each end of wash coat, and provided at each end with an electrical contact. This example is designed for a 12 volt battery system. The whole assembly is then folded, preferably in an accordion fold, an insulator disposed between contiguous reaches of the fold to prevent shorting or welding, and then placed in a suitable housing.
This structure is particularly advantageous in enabling the entire unit to draw sufficient power from a 12 volt or 24 volt automotive system to bring the monolith to a desired temperature e.g., 600.degree. F., very quickly. Because of the parallel path, all touching points of the layers are at nearly the same potential and shorting between touching contiguous layers is avoided. The plural paths enable the device to draw more power than prior devices without shorting and to achieve a higher temperature more quickly. Thus, the unit is capable of accomplishing the desired preliminary temperature elevation to catalyst operating temperature until such time as the exhaust is able to heat, and oftentimes maintain, the entire converter at optimum operating temperature.